We have identified a cellular tumor antigen of 54,000 MW. This protein is in high levels in a variety of transformed and tumorigenic cells and in low levels in normal cells. Similar proteins have now been identified in human, monkey, mouse, rat and hamster cells transformed with a wide variety of agents; i.e.-DNA or RNA tumor viruses, chemicals, X-rays, spontaneous or genetically predisposed tumors. The m-RNA for this protein can be assayed by in vitro translation of this m-RNA with the resultant production of a 54,000 MW protein in vitro that is immunoprecipitated by antibody to the in vivo protein and has a similar or identical peptide map with the in vivo protein. While the levels of the 54,000 MW protein in virus infected or transformed cells is 100 fold greater than the levels of this protein in normal cells, the levels of the 54,000 MW protein m-RNA are equal in normal and transformed cells. The way in which the 54,000 MW protein appears to be regulated is at the level of protein turnover. Pulse-chase experiments have demonstrated that the half life of the protein in normal cells is about one half an hour while in SV40 transformed cells the half life of the 54,000 MW protein is greater than 24 hours. Similarly in tumorigenic embryonal carcinoma cells the half life of the 54,000 MW protein is about 3-6 hours while in cells differentiated from these embryonal carcinoma cells, which are no longer tumorigenic, the half life of the protein drops to less than 30 minutes.